Note that this script does not correctly display whether a moon is in front of or behind Jupiter. Further, it does not account for the movement of Earth nor for the various inclinations of the orbital planes. This may result in slight but significant differences in position.
Sky and Telescope
Shallowsky.com
With a little math, we can calculate where Galileo saw Jupiter's moons more than 4 centuries ago. Check out Earnie Wright's webpage!